This invention relates generally to filter devices useful for trapping blood clots and controlling embolization and thrombosis in blood vessels. More specifically, the present invention is directed to a blood clot filter, which has improved self-centering capabilities.
Blood clots (emboli) which are carried in the blood stream often pose serious threats to a person""s health and may lead to death. For that reason, the reduction of such clots and their stabilization and arrest against further migration from the vena cava into the pulmonary circulation are necessary.
A number of types of permanent filters have been designed for this purpose. Generally, these filters are in the form of a frustoconical basket which is attached to the interior of a vein downstream of the area sought to be filtered. One type includes a plurality of wire legs of the same length extending in a generally conical array from a common apex to their free ends which attach to the wall of the vessel, thus forming a cone of revolution with a single plane of contact with the vessel wall. A filter of this type is described in U.S. Pat. No. 3,952,747. The filter shown therein includes sharpened hooks at the free wire ends to permanently anchor the filter by impaling the hooks in the blood vessel. If the hooks engage the cava in a plane perfectly perpendicular to the vena cava, the filter, in theory, would be centered. In reality such perfect deployments rarely occur and so the filter is often deployed in a tipped position, with its apex not coincident with the apex of the vessel.
In an effort to facilitate the correct positioning of these permanent types of filters, U.S. Pat. No. 4,688,553 provides for a self-centering filter where at least some of the legs are, towards their free ends, provided with appendices which extend substantially parallel with the substantially cylindrical wall of the vessel when the filter is inserted within the vessel and whereby each appendix includes a portion extending in the general direction of the apex from which the legs radially diverge and up the vena cava wall. It is the appendices which allow for centering of the filter inside the vein. A disadvantage of this filter is its inability to be easily retrieved. As the appendices of these filters extend in the direction of the apex, the ends thereof would increase interference with the venal wall upon removal thereby rendering removal more difficult.
A general disadvantage of the conical filters of the type described above is that the flexibility of the wires prevents consistent bearing forces from being applied to the vein wall. Moreover, the wire is small, and the bearing surface in contact with the vein is restricted which can lead to trauma and perforations.
Furthermore, filters formed from a conical portion of wire legs are generally difficult to load before emplacement due to the interference of the hooks with each other when the filter is folded into the ejector used for emplacement. Moreover, once the filter is ejected and implanted, it cannot be removed. This often results in permanent placement of the filter in an undesirable location and often requires the placement of another filter at the appropriate location.
Once the thrombotic condition is resolved, typically in a period of 6 weeks to six months, the filter is removed. Since filters of the permanent-type described above are difficult to remove, retrieval is only attempted when medically necessary.
A number of conical filters have since been designed which are both retrievable and self-centering. For example, U.S. Pat. No. 5,152,777 describes a self-centering filter with resilient filter wires designed for temporary emplacement within a vessel. The filter is of the same general conical design as the aforementioned filters except that the filter wires have rounded tips instead of hooks at their free ends to facilitate removal. Moreover, a longitudinally stable stem is permanently affixed to the collar at the vertex of the filter. Once emplaced in the body, the stem remains attached to the filter and serves to keep the -filter centered in the vessel and to facilitate retrieval. This filter therefore has a separate centering portion, consisting of the stem, and a separate trapping/filtering portion consisting of the conical filter wire portion.
A disadvantage of this filter is that it is not firmly anchored to the vessel wall due to the rounded tips at the free wire ends. The filter, even when open, may be moved horizontally within the vessel. This can allow clots to escape the filter. Yet another disadvantage of this filter is that because the stem needs to remain for centering purposes, the patient""s immune system suffers the burden of the presence of a larger foreign body. In addition, an obturator needs to be inserted in the lumen of the stem to prevent stagnant blood from accumulating in the lumen and forming potentially dangerous emboli.
It is desirable, therefore, to provide a self-centering vena cava filter which is firmly anchored and retrievable and which does not require a separate centering portion. Moreover, it would be desirable to provide a filter wherein the bearing force is distributed over a larger surface of the venal wall in order to reduce trauma and perforations thereto and to facilitate retrieval of the filter. Finally, it would be desirable to provide a filter which has a reduced profile and which is easier to load into an ejector and to implant within a vein.
It is an object of the present invention to overcome the disadvantages of the aforementioned filters by providing a retrievable filter that has improved self-centering capabilities in the human vena cava without the need for a separate centering portion and which provides secure anchoring of the filter to the vena cava wall.
It is a further object of the present invention to provide a filter which reduces the incidence of collapsing of the cava, decreases trauma owing to contact pressure on the vein walls and reduces the force necessary to retrieve the filter.
The present invention provides a two-planar filter with improved centering capabilities due to its two planar points of contact with the inner venal wall. Contact at two planes is provided by the contact of two sets of filter wires wherein each set differs in length from the other. In one preferred embodiment, centering capability is additionally improved by providing at least some of the filter wires, preferably in the set of wires of lesser length, with extensions. Barbs, e.g. hooks, are attached to at least one set of filter wires to ensure firm anchoring to the venal wall. Having two planar points of contact has the added benefit of reducing the profile of the filter, which improves ease of emplacement and allows for insertion in veins of lesser diameters. In addition, contact at two different planes reinforces the walls of the vena cava, thus reducing collapse of the vein. Two-planar contact also reduces the trauma to the venal wall owing to contact pressure on the vein walls and reduces the force necessary to retrieve the filter. Lastly, the filter design is versatile in that it allows for filter wires which are straight, corrugated or spiral in design, as well as filter wires which have any combinations of straight, corrugated or spiral portions intermediate their length. Each design presents its own advantages.